1. Field of the Invention
The invention relates to an absorber tube.
2. Description of Related Art
DE 10 231 467 B4 describes such an absorber tube which is used in particular for parabolic trough collectors in solar power stations. The absorber tube has a central metal tube and a glass sleeve tube surrounding the central metal tube. The glass sleeve tube is provided on both ends with a glass-metal transition element, on each of which an expansion compensating device or an attachment element engages. The expansion compensating device is disposed at least partially in the annular space between the metal tube and the glass transition element.
The attachment element can extend in the annular space between the expansion compensating device and the metal tube, so that with a conical configuration of the attachment element the directly impinging radiation as well as the radiation from the metal tube is reflected back onto the metal tube.
The attachment element can also extend in the annular space between the expansion compensating device and the glass sleeve tube and in this case is connected to the glass-metal transition element. The attachment element has a fastening element in the form of an annular disc by which the attachment element is fastened on the expansion compensating device which may comprise a bellows. An annular disc is a flat round object.
DE 60 223 711 T2 discloses an absorber tube in which the glass-metal transition element and the expansion compensating device are disposed in the configuration of a bellows one behind the other in the axial direction. On the outer face are provided a first shield element, which shields the bellows, and a second shield element, which shields the glass-metal transition element against incident radiation. Moreover an internal radiation shield is disposed in the annular space between the glass sleeve tube and the metal tube in the region of the glass-metal transition element. The internal radiation shield is attached in the bellows by retaining arms.
A disadvantage of this arrangement with bellows and with a glass-metal transition element is the relatively large overall length, which leads to a reduction of the free aperture and thus of the efficiency.
The glass-metal transition element is protected at the outset by one of the two outer shield elements against direct irradiation from the exterior. The internal radiation shield having an L-shaped cross-section only has support arms in the region of the glass-metal transition element, so that direct incident and reflected radiation from the metal tube can impinge on the glass-metal transition element in the region between the support arms. Only a part of this radiation is caught by the internal radiation shield.
This design has the further disadvantage that it absorbs radiation itself and as a result is heated. The internal radiation shield is only insufficiently thermally coupled to the bellows by means of the support arms, so that the proportion of the radiation which is absorbed by the annular component must for the most part be emitted again by radiation. A considerable proportion of the heat radiation strikes the glass-metal transition element. As a result the glass-metal transition element undergoes a secondary heat input by radiation from the heated radiation shield.
Both DE 10 231 467 B4 and also DE 60 223 711 T2 disclose an externally located protection of the glass-metal transition and the expansion compensating device. Nowadays this externally located protection is generally fitted after the installation of the receiver in the power station. For this purpose shielding plates are fitted after the receivers are welded together in the field.
A disadvantage in this case is that the glass-metal transition is unprotected during transport and installation and as a result can be easily damaged. During the fitting process scratches can occur in the sleeve tube, wherein scratches in the vicinity of the glass-metal transition element are particularly critical and can lead to weakening of the absorber tube and to subsequent breakage of glass in operation.